1. Field of the Invention
The invention relates to methods of inhibiting tumor cell proliferation by inhibiting FoxM1B activity. Specifically, the invention relates to methods of inhibiting tumor cell proliferation by inhibiting FoxM1B activity, expression, or nuclear localization in a tumor cell. The invention also relates to screening methods for identifying compounds that can inhibit tumor cell growth by inhibiting FoxM1B activity, expression, or nuclear localization in a tumor cell.
2. Background of the Related Art
The Forkhead box transcription factors have been implicated in regulating cellular longevity and proliferative capacity. Such studies include a finding of increased longevity in C. elegans bearing a mutant daf-2 gene, which encodes the worm homolog of the insulin/Insulin-like Growth Factor 1 (IGF1) receptor (Lin et al., 1997, Science 278: 1319-1322; Ogg et al., 1997, Nature 389: 994-999). Disruption of the daf-2 gene abolishes insulin-mediated activation of the phosphatidylinositol 3-kinase (PI3K)—protein kinase B/Akt (Akt) signal transduction pathway and prevents inhibition of the forkhead transcription factor daf-16 (corresponding to mammalian homologs FoxO1 or Fkhr) (Paradis and Ruvkun, 1998, Genes Dev. 12: 2488-2498). Activation of the PI3K/Akt pathway phosphorylates the C-terminus of the Daf-16 (FoxO1; Fkhr) gene product and mediates its nuclear export into the cytoplasm, thus preventing FoxO1 transcriptional activation of target genes (Biggs et al., 1999, Proc. Natl. Acad. Sci. USA 96: 7421-7426; Brunet et al., 1999, Cell 96: 857-68; Guo et al., 1999, J. Biol. Chem. 274: 17184-17192).
More recent studies of Daf-2− C. elegans mutants have demonstrated that Daf-16 stimulates expression of genes that limit oxidative stress (Barsyte et al., 2001, FASEB J. 15: 627-634; Honda et al., 1999, FASEB J. 13: 1385-1393; Wolkow et al., 2000, Science 290: 147-150) and that the mammalian FoxO1 gene could functionally replace the Daf-16 gene in C. elegans (Lee et al., 2001, Curr. Biol. 11: 1950-1957). In proliferating mammalian cells, the PI3K/Akt signal transduction pathway is essential for G1 to S-phase progression because it prevents transcriptional activity of the FoxO1 and FoxO3 proteins, which stimulate expression of the CDK inhibitor p27kip1 gene (Medema et al., 2000, Nature 404: 782-787). Moreover, genetic studies in budding yeast demonstrated that forkhead Fkh1 and Fkh2 proteins are components of a transcription factor complex that regulates expression of genes critical for progression into mitosis (Hollenhorst et al., 2001, Genes Dev. 15: 2445-2456; Koranda et al., 2000, Nature 406: 94-98; Kumar et al., 2000, Curr. Biol. 10: 896-906; Pic et al., 2000, EMBO J. 19: 3750-3761).
Forkhead Box M1B (FoxM1B) transcription factor (also known as Trident and HFH-11B) is a proliferation-specific transcription factor that shares 39% amino acid homology with the HNF-3 winged helix DNA binding domain. The molecule also contains a potent C-terminal transcriptional activation domain that possesses several phosphorylation sites for M-phase specific kinases as well as PEST sequences that mediate rapid protein degradation (Korver et al., 1997, Nucleic Acids Res. 25: 1715-1719; Korver et al., 1997, Genomics 46: 435-442; Yao et al., 1997, J. Biol. Chem. 272: 19827-19836; Ye et al., 1997, Mol. Cell Biol. 17: 1626-1641).
In situ hybridization studies have shown that FoxM1B is expressed in embryonic liver, intestine, lung, and renal pelvis (Ye et al., 1997, Mol. Cell Biol. 17: 1626-1641). In adult tissue, however, FoxM1B is not expressed in postmitotic, differentiated cells of the liver and lung, although it is expressed in proliferating cells of the thymus, testis, small intestine, and colon (Id). FoxM1B expression is reactivated in the liver prior to hepatocyte DNA replication following regeneration induced by partial hepatectomy (Id).
FoxM1B is expressed in several tumor-derived epithelial cell lines and its expression is induced by serum prior to the G1/S transition (Korver et al., 1997, Nucleic Acids Res. 25: 1715-1719; Korver et al., 1997, Genomics 46: 435-442; Yao et al., 1997, J. Biol. Chem. 272: 19827-19836; Ye et al., 1997, Mol. Cell Biol. 17: 1626-1641). Consistent with the role of FoxM1B in cell cycle progression, elevated FoxM1B levels are found in numerous actively-proliferating tumor cell lines (Korver et al., 1997, Nucleic Acids Res. 25: 1715-1719; Yao et al., 1997, J. Biol. Chem. 272: 19827-36; Ye et al., 1997, Mol. Cell Biol. 17: 1626-1641). Increased nuclear staining of FoxM1B was also found in human basal cell carcinomas (Teh et al., 2002, Cancer Res. 62: 4773-80), suggesting that FoxM1B is required for cellular proliferation in human cancers.
These studies and others suggest that FoxM1B plays some role in human cancers. FoxM1B, therefore, would provide an attractive target for anti-cancer therapies because FoxM1B expression typically declines during normal aging (see co-owned and co-pending U.S. provisional patent application, Ser. No. 60/426,068, filed Nov. 13, 2002, incorporated by reference herein). Thus, FoxM1B might provide a selective target that is more active in tumor cells than in normal cells, particularly terminally-differentiated, aged or aging normal cells that surround a tumor, allowing tumor cells to be treated while minimizing the deleterious side-effects of such compounds on normal cells.